Standard fishing rods, often called duct rodders, are known for pulling back cables or optical fiber cables to be installed in a long conduit. These rods in general comprise one elongated cylindrical main body or several cylindrical elongated parts to be connected at the required length. The elements are usually unwound from a coil and pushed in the conduit into which the cable will be pulled in return. Since the conduits may present bends or junctions, the elongated parts shall present flexibility to pass these bended portions. The problem linked to the aforementioned flexibility is that the maximum insertion length achievable is limited by undulations of the rod inside the duct. The required force to push the rod inside the conduit increases with the inserted length of the rod. The higher the pushing force, the shorter the induced undulation periods of the rod inside the conduit, leading to increased friction forces, and above a certain critical limit even rendering impossible any further movement of the rod inside the duct.
The pushing force depends on at least two different factors: the friction of the rod due to its weight and induced undulations in the straight portions of the duct and the friction induced by the reaction of the rod in the bends or junctions.
Concerning this friction of the rod, it is possible to reduce it with the use of lubricant or fluid injected in the conduit simultaneously with the rod. These techniques imply using sophisticated machines, increase the time to set up the installation, require the supply of fluid (oil, water or compressed air), leading to an increase of the costs of pushing the rod into the conduit.
Concerning the additional friction forces coming from bends or junctions, some attempts have been carried out to facilitate the introduction of the rod through these obstacles. For example, the document U.S. Pat. No. 4,895,221 discloses a duct rod equipped at its head with a foam saturated with oil. When the head enters a bended portion of the duct, the foam releases its oil and the friction between the duct walls and the rod is reduced. However, this apparatus may not allow passing numerous bends, as the oil saturated in the foam will be released rapidly in the first bends. Another apparatus is disclosed in the document US2004180194A1. This document discloses a duct rod or fish tape with a rolling head. The rolling head helps the rod to pass the bends of the conduit, but it should be noticed that nothing with this apparatus will reduce the pushing force after the head has passed the bend, as the rod will rub against the conduit wall along the bend. Focusing on junctions, EP2194625 discloses a robot designed to help choosing the correct way when the rod head is at a junction. The robot is able to articulate to follow the right way. However, nothing will decrease the required pushing force after the rod head will be introduced in the correct conduit, as, like to the aforementioned situation in the bends with the rolling head, the rod will rub against the wall conduit. All the above mentioned points are limiting the possibility to push a duct rod through long conduits.